Report check runs in a batch

Upserts up to 50 of the caller's checks on one exact commit OID in one transaction, each as Report a check run would, and returns one outcome per item in request order. A malformed body, a repeated key, or more than 500 annotations or 1 MiB of output and annotation text, paths included, in all (check-batch-too-large) refuses the whole batch; an item GitCafe declines (refused, with its error) does not stop the others. Requires authenticated authority and enforces current resource policy.